Among others, heat exchangers are important apparatuses that are widely used in petrochemical industry, energy power industry, metallurgical industry, refrigeration engineering and seawater desalination. Among heat exchange equipments, the shell-and-tube heat exchangers are predominant, accounting for about 55-70%. This type of heat exchanger has a simple structure that mainly contains two parts, i.e., heat exchange tube bundles and shells. When one kind of fluid flows inside the tubes, and the other kind of fluid flows outside the tubes against the shell side, the two fluids indirectly exchange heat through the tube wall.
In a shell-and-tube heat exchanger, a more important function of the baffles, besides supporting the tube bundles, is to change the flow direction of fluid in the shell-sides so as to enhance heat transfer rate.
There exist many problems in the conventional segmental baffles, e.g., (1) a high pressure drop occurs since the segmental baffles make fluid perpendicularly impact the shell wall and the tubes, leading to an increased power load; (2) the fluid with high speed crosses the heat exchange bundles laterally, inducing vibrations of heat exchange tubes and thus a reduced service life; (3) the heat transfer rates decrease due to a flow stagnation region generated at the joint of baffles and shell walls, where fouling tends to accumulate as well; and (4) the mass flow rate laterally crossing tube bundles is efficiently decreased due to the bypass flows and leaking flows which exist between baffles and shell walls and between heat exchange tubes and baffles, resulting in a reduced heat transfer rate on the shell side.
Aimed at the above problems, some new kinds of shell-and-tube heat exchangers with helical baffles are developed in recent years. In these newly developed heat exchangers, baffles are arranged in helix to make the fluid on the shell side of the heat exchanger flow along a helical path, resulting in an affirmative reduction in flow pressure drop on the shell side and an enhancement in heat transfer rate. Heat exchangers with helical baffles in the prior art may be classified into two categories, one being heat exchangers with non-continuous helical baffles employing non-continuous helical baffles formed of a plurality of fan or oval shaped flat plates, with the non-continuous helical baffles in a continuously overlap form (see CN Patent Application No. 99241930.1 and U.S. Pat. No. 6,827,138 B1,) or in a staggered helical form (see CN Patent Application No. 200320106763.1); the other being heat exchangers with continuous helical baffles employing continuous helix (see CN Patent Application No. 200510043033.5). As compared with non-continuous helical baffles, the continuous helical baffles make the flow assume a helical pattern, which further reduces pressure drop and leakage. However its manufacture is more complicated than in the case of non-continuous helical baffles. This is especially the case when the pitch is large, that is, the helical surface becomes relatively steep in portions close to the central axis, which makes it more difficult, or even impossible to manufacture curved surfaces and to position and form holes on these surfaces. Currently, in order to make it easier for fluid on the shell side to accomplish helical flow patterns, most of shell-and-tube heat exchangers with continuous helical baffles are additionally installed with a central tube of a certain diameter along the centre axis. This somehow mitigates the difficulty in the manufacture of continuous helical baffles, however, it relatively decreases the efficient heat exchange area of heat exchangers since no fluid passes through the central tube, and the diameter of the central tube increases with the increase of baffle pitch.
Moreover, some researches show that, given same tube-side arrangements and same shell-side flow rate, the current single shell-pass heat exchanger with helical baffles has higher heat exchange capacity under the same shell-side pressure drop. While its pressure loss is lower than that of a traditional heat exchanger with segmental baffles, however, its heat exchange capacity is also lower, simultaneously which can hardly meet users' requirement. To enhance shell side heat transfer rate, a multiple shell-pass shell-and-tube heat exchanger with continuous helical baffles was proposed (see CN Patent Application No. 200610041949.1, which corresponds to Publication No. CN1821700A). Given same number of tube-side and same flow rate, the velocity of fluid in a shell-side in a multiple shell-pass shell-and-tube heat exchanger is higher than that in a single shell-pass shell-and-tube heat exchanger. Therefore the heat exchange coefficient becomes higher, that is, a higher heat transfer rate is achieved.
A non-continuous helical baffle is formed by splicing a plurality of fan shaped or oval shaped flat plates. This has an advantage that manufacture is easy. Generally, a central pole is employed for positioning the center and the volume occupied by the central pole is small. However, there is a relatively large leakage, which affects heat exchange. Continuous helical baffles are formed by splicing complete continuous helical baffles of many cycles, each cycle being a continuous helical curved plate, such that the flow behavior approximates to a helical pattern. This has an advantage that pressure drop and leakage are reduced and heat transfer coefficient is higher, however, when the pitch is large, the helical surface becomes relatively steep at portions close to the central axis, where it is difficult to manufacture the continuous helical surfaces. Generally, a central tube is employed to fit the helical structure inside of the helix. However, as heat exchange tubes can not be arranged at the location of the central tube, the effective heat exchange area of the heat exchanger is relatively decreased, and part of the heat exchanger volume is occupied, thus leading to a decreased compactness. Currently, there is no heat exchanger with helical baffles having the advantages of both continuous helical baffles and non-continuous helical baffles.
Further, the shell-and-tube heat exchangers used in industries are generally in form of a horizontal type. The continuous helical baffles may reduce leakage, however when the fluid on the shell side is such a medium that tends to foul, fouling can accumulate at the bottom of the horizontally arranged shell-and-tube heat exchanger due to a low flow rate. Especially when the helical angle is small, a large amount of fouling will deposit and cleanup becomes difficult, thus resulting in a decreased heat transfer rates.